1. Field of the Invention
The present invention relates to electrochemical probes, and more particularly to electrochemical sensors for measuring certain characteristics of aqueous liquids, and components of such sensors.
2. Description of the Related Art
In many situations, it is desired to measure and to monitor a variety of characteristics of liquids, especially aqueous liquids, and often it is desirable to monitor such characteristics at frequent intervals—or even continuously. Such characteristics include the pH of the liquid, the total dissolved solids in the liquid, the temperature of the liquid, the concentration of ionic solutes such as chlorine species in the liquid, and the oxidation reduction potential (ORP) of the liquid (that is, the relative tendency of materials in the liquid to undergo oxidation or reduction—particularly the ability of the liquid to destroy bacteria in it). Many of these characteristics can be measured electrolytically.
Measurement of a liquid's characteristics such as pH, halide concentration, ORP and solute concentrations are of interest in a wide variety of industrial, commercial and domestic processes and situations. For example, many chemical processes are pH-, ORP- or solute-dependent. The pH, ORP or solute levels of effluents from factories are commonly of environmental concern.
The pH, halide concentration, ORP or solute level of water in particular is critical in many settings. For example, a setting familiar to many laypersons is in various water storage systems, such as air conditioning systems, swimming pools and spas. Therefore, for ease of explanation, much of the following discussion will be with reference to the swimming pool setting, although it should be borne in mind that the discussion is likewise applicable to any other situation in which the pH, halide concentration, ORP and/or solute level of a liquid is of interest.
In a swimming pool, the quality of the water is closely related to the pH and ORP of the water. In swimming pools, the ORP of the water is a measure of the free chlorine level in the water, and the solute level may be of chlorine species resulting from addition of, for example, hypochlorous acid or sodium hypochlorite, the concentrations of free chlorine and other chlorine species being related to the biological antiseptic quality of the water.
Therefore, the pH and concentration of chlorine species (“chlorine level”) of swimming pool water must be monitored to ensure that an adequate water quality level is maintained. Conventionally, this is carried out by hand and the owner or other caretaker in charge of maintaining the pool must repeatedly go to the pool with vials and chemicals, scoop out the water into the vials, shake the vials, compare the colors of the resulting solutions to those on charts coordinated with the pool volume to determine the amounts of chemicals to add to restore the proper pH and/or chlorine level, obtain those chemicals, measure them out and add them to the pool. Not only is this a cumbersome process, but if it is not carried out at frequent enough intervals, the quality of the pool water can become unacceptable very quickly. Thus, for example, if the pool caretaker is away for a several days, he or she may return to find a pool filled with murky water. Or the pH or chlorine level may fall out of acceptable range too soon before the next testing and the water may become unhealthful, and the pool caretaker may not realize that fact until it is too late. Therefore, it is desirable to have a pH monitor and/or an ORP monitor that would carry out the measurement task automatically and frequently even continuously—on a real time basis.
Sensors for such measurements often operate on a potentiometric electrochemical principle that incorporates a reference electrode and a sensing electrode. Conventional reference electrodes for use in such potentiometric electrochemical measurements typically incorporate an internal reference fill solution in contact with an electrode in contact with a test solution through a porous junction, which allows a slow leak of the internal reference fill solution to provide the necessary electrolytic contact with the liquid being tested. A metal or electrochemical electrode in contact with the test solution completes the circuit and an electrical potential on the reference electrode remains relatively constant while the sensing electrode responds to chemical changes in the test solution.
Conventional electrodes of this type suffer from several drawbacks when applied to certain measurement environments, such as long-term unattended monitoring of pool or spa water. For example, because leakage of the internal reference fill solution through the porous junction into the tested environment is necessary to provide electrolytic conductivity between the internal reference fill solution and the tested environment, the useful life of the reference electrode is limited. Moreover, a high rate of leakage is desirable to produce a low electrical impedance of the reference electrode. Moreover, in a pipe mounted system, the flow of test solution flowing over the electrode exacerbates the high leakage rate. Thus, while low electrical impedance is desirable for accurate measurements because it reduces noise, the high leakage rate employed in such conventional reference electrodes to produce the desired low electrical impedance severely limits the life of the electrode and the electrode must be frequently refilled with fresh internal solution or replaced.
Such conventional reference electrodes suffer from other disadvantages as well. For example, they tend to be fragile, typically being encased in glass. Moreover, they often are limited in operational orientation. In other words, because the reference fill solution of the electrode is a liquid, it readily flows as a result of gravity. Thus, the relative orientation of the electrode with respect to the reference fill solution and the reference fill solution with respect to the porous junction depends on the spatial orientation of the electrode and so the electrode assembly in the test solution must be oriented vertically so that the reference fill solution is properly oriented in the electrode. Indeed, silver/silver chloride reference electrodes suspended in glass-encased fill solutions have been employed in combination with antimony electrodes in some pH sensors, with all the attendant disadvantages of glass membranes and fill solutions noted above.
Reference electrodes according to the manufacturer of the reference electrode described in U.S. Pat. No. 7,462,267 are also available. They use a gelled polyelectrolyte internal fill solution to reduce the osmotic ionic pressure resulting from the high ionic concentration in the internal fill solution relative to the low ionic content of surrounding solution. Missing from the discussion was the impact of the porosity of the frit used to separate the internal fill solution from the environment and the necessity of mechanical fixture of the porous frit. Such electrodes were procured and evaluated and found not only to display a marked sensitivity to changes in the amount of chloride based salts in solution but due to the high porosity (35 microns) that the fill solution would leach out during regular use over a short time from (2 months). Also with regard to the placement of the frit it was found that during shipment that differences in atmospheric pressure routinely push the frit from the electrode body resulting in complete loss of electrolyte.
Potentiometric solid state sensors, in which measurement is based on ion specific voltage developed between a reference electrode and a measuring sensor, have been described in scientific publications, but their commercialization is extremely limited, and no large-scale production process has been developed. In any event, they also suffer from other serious disadvantages. They require frequent calibration, they suffer from significant drift, they have very limited lives, have limited applicability because some common salts damage their sensor membranes, are expensive (although, at about US $20, are less expensive than the previously mentioned meters), are affected substantially by temperature, and while they can be stored dry, they require about 24 hours for stabilization.
U.S. Pat. No. 5,497,091 describes a pH sensor that employs an antimony electrode in combination with a ceramic reference electrode, but provides no clear description of the ceramic reference electrode. However, known antimony-based pH sensors typically employ a polished antimony surface for enhanced sensitivity and so suffer from deteriorating sensitivity as they lose polish. In addition, known antimony-based pH sensors typically also suffer from substantial drift as the quality of the polish diminishes over time, and, as with the other pH sensors dependent on ion exchange between the fluid being tested and the fill solutions, known antimony-based pH sensors frequent recalibration and limited lifetimes due to gradual dilution of the fill solution. Moreover, because they employ a liquid fill solution, they must be maintained in a vertical position. Thus, state of the art reference electrodes for pH monitors are unsuitable for many uses, especially those in which a durable, inexpensive and readily available (or easy to manufacture) probe for frequent, accurate, real time pH measurements with low drift.
Monitors for measuring ORP also are available commercially. Their measurements are based on a voltage developed between a silver/silver chloride reference wire in an internal fill solution and a platinum wire isolated from the fill solution. Such monitors also suffer from serious drawbacks, including high cost (about US $250), unavailability in large quantities, limited life based on the fill solution, lengthy response time, the requirement of a soaker cap to keep the tip wet when not in use, and the requirement of all electrodes that employ fill solutions that that they be maintained in an almost vertical position to keep the electrodes in the tested fluid. Thus, as with the state of the art pH monitors, state of the art reference electrodes for ORP probes are unsuitable for many uses, especially those in which a durable, inexpensive and readily available (or easy to manufacture) probe for frequent, accurate, real time ORP measurements with low signal drift.
The reference electrode is paired with a sensing electrode correlated with the characteristic to be measured. As noted above, such characteristics may include the concentration of sanitizer such as chlorine and the total dissolved solids in the liquid.
With respect to the sanitizer level, it should be noted that, as mentioned above, many aquatic environments, such as pools and spas, contain certain chemicals to sanitize the water and to maintain its quality against a variety of organisms and other organic matter in it. A common approach to water treatment is to use some sort of oxidizing chemical to remove bacteria and impurities from it. For example, frequently, the oxidizing chemical employed is one that imparts a halogen such as bromine or, more commonly, chlorine to the water. Thus, the discussion below will often be directed specifically to chlorine, but it should be understood that it may be applied to other halogens as well.
Typically, in a swimming environment, chlorine is added until all the contaminants are oxidized with some “residual” amount left over to oxidize more over the course of time. This is referred to as “free chlorine.” This residual amount establishes a sufficient chlorine level in the water to continue to oxidize (sanitize) more bacteria as they are produced and it is desired that the concentration of the halogen in the aquatic environment be maintained within a relatively narrow range so that the concentration is high enough to sanitize the water, but not so high as to be either wasteful or undesirable for human interaction. A common standard in pools is to keep the free chlorine between about 1 and about 3 parts per million (ppm) (as used herein, ppm measurements, are on a weight basis). In spas, the free chlorine leval is somewhat higher; generally about 3 to about 5 ppm.
Therefore, it is important to have an accurate way to carry out frequent or regular measurements of the level of halogen such as free chlorine in liquid aqueous environments, especially levels that are on the order of from about 1 to about 3 ppm. Unfortunately, however, the chemicals employed to sanitize the water interact with efforts to measure the halogen content of the water and so it is difficult to measure the halogen concentration accurately.
Further complexity is added by the characteristics and nature of the chlorine that is employed. In more detail, and with specific reference to the use of chlorine in swimming pools and spas, chlorine used to treat swimming water exists in two basic forms, unstabilized and stabilized. Common types of unstabilized chlorine are household bleach and granular sodium hypochlorite. Unstabilized chlorine is chemically unstable in water exposed to sunlight, especially direct sunlight. Sunlight breaks down chlorine over time to a point where it no longer has a sanitizing effect on the water. Indeed, the effect of sunlight on unstabilized chlorine can be quite dramatic. For instance, the chlorine level of water treated with unstabilized chlorine can be reduced from several ppms to almost 0 by exposure to direct sunlight for a half an hour at moderate temperatures. Therefore, water containing unstabilized chlorine as a sanitizer requires regular addition of chlorine to maintain sufficient chlorine levels.
Stabilized chlorine, on the other hand, is chemically modified to last longer in sunlight. Thus, the chlorine used to treat outdoor pools is almost exclusively stabilized chlorine. Common types of stabilized chlorine are trichloro-n-triazine trione tablets and sodium dichloro, in granular and tablet form. Unstabilized chlorine can also be stabilized by adding granular cyanuric acid to the water along with it. Accordingly, cyanuric acid is a common ingredient in stabilized chlorine. Cyanuric acid binds chemically to some of the chlorine holding it in reserve to slow its rapid deterioration in sunlight.
By contrast, indoor pools generally are not exposed to direct sunlight and so most indoor pools do not require use of stabilized chlorine. There are benefits in not using stabilized chlorine and pure cyanuric acid in particular. For example, cyanuric acid, a form of which all forms of stabilized chlorine contain, is toxic and in the United States, mandated limits have been instituted on the amount of cyanuric acid a pool may contain; typically about 140 ppm. Secondly, the only widely available test for cyanuric acid is a turbidity test that is subject to substantial error.
The presence of cyanuric acid also creates substantial difficulty in the determination of active chlorine levels in water. This difficulty in water containing cyanuric acid arises from the limitations of the various methods used to measure chlorine and the notion of “free” or residual chlorine. Of the various test methods for determining chlorine concentration, the DPD (N,N-Diethyl-p-phenylenediamine) test is the most common. Its use has become common because of the expense and general unavailability of electronic instruments needed by other methods.
The DPD test involves taking a sample of water and adding a reagent to it to effect a color change in it, then adding another reagent drop-wise to remove the color. The amount of residual chlorine is related to the number of drops required to remove the color from the sample. This test, however, has several drawbacks. For example, there is a limit as to how much of the de-coloring reagent can be added after the initial color change reagent.
In fact, depending on the level of chlorine in the water, two tests may be required. If, after adding the color change reagent, the sample is extremely dark, dilution is required and so a smaller volume of sample is used for the second test. Also, the chemicals used have a finite shelf life, requiring the chemicals to be replenished. Chemical test strips based on this same color-change test mechanism based on the amount of chlorine to which they are exposed also may be used. The resulting color of the test strip is compared to a chart of colors that indicate a general range of chlorine associated with it. It is very difficult to pinpoint at levels around about 3 ppm by color matching, however, and so such tests are only a general indication of the amount of chlorine present. Moreover, the test strips also have a limited shelf life. Other tests employ an electronic sensor that generates a signal based on the chemical activity of free chlorine. The presence of cyanuric acid in the water interferes with the results such tests and so the results of measuring the level of free chlorine in water differ depending on whether or not the water contains cyanuric acid. Therefore, the presence or absence of cyanuric acid poses a serious challenge to the measurement and control of chlorine levels in a pool environment.
In some elaborate pool installations, the chlorination of the pool system is controlled automatically with an electronic chlorine sensor mounted inline with a chlorine-dispensing mechanism. When and how much chlorine is added is determined by set points in the controller based on the output of this sensor. The initial set point is typically based on a reading from a DPD test.
However, a difficulty arises if stabilized chlorine is added to the water after initial setup. The degree to which the stabilized chlorine is a problem in chlorine measurement depends at least in part on the amount of cyanuric acid it introduces into the water and the type of electronic sensor used to measure the chlorine. Electronic sensors measure the actual real-time amount of free chlorine active in the water, but since cyanuric acid binds with some of the chlorine, the bound chlorine is not free and active and contributes only minimally to the electrical signal. Therefore, it remains largely unmeasured and if any cyanuric-based chlorine is introduced into the water after this there will be a difference between the results of a DPD test and the measured ppm using an electronic sensor. The DPD test will indicate a higher measurement of free chlorine than will the electronic sensor. This is because the reagent added to the water sample used by the DPD test to effect the color change effectively releases the bound chlorine from the cyanuric acid, allowing it to be measured.
If the DPD test is used to set the system up without cyanuric acid, the two results will be the same. However, if cyanuric acid is added later the displayed amount of chlorine on the electronic system will drop with respect to the DPD measurement and, depending on the set point, possibly start erroneously adding chlorine to the pool. This effect cannot be compensated for in a systematic way because the effect is variable depending on the relative levels of cyanuric acid and chlorine and how much sunlight there is at the pool site, Nevertheless, it is still desirable to maintain at least a small amount of cyanuric acid for, without it, the chlorine breakdown cause(d) by the exposure to direct sunlight would produce an extraordinary and costly chlorine demand.
Several types of electronic sensors are used in automated measurement systems. The most common is an oxidation reduction potential (ORP) sensor, an example of which is discussed in U.S. Pat. No. 4,224,154 to Steininger. This type of sensor employs an electrode that is most affected by cyanuric acid.
The sensing mechanism of the sensor is based on the fact that adding an oxidizer such as chlorine to water generates a voltage between two electrodes of the sensor. One of the electrodes is an unpolarized reference electrode. The other electrode is a catalytically reactive noble metal such as platinum. The voltage generated between the electrodes is based on the simultaneous oxidation of impurities and the reduction of chlorine occurring in the process. The voltage output from this type of sensor is between 0 and about 900 millivolts. However, the response of the electrode is logarithmic; i.e., the greatest change in the output voltage occurs within the first 2-3 ppm of added chlorine with almost no correlation at higher levels.
Typically, an algorithm is used to relate the ORP signal to a ppm reading. However, the sensor's logarithmic response can produce wide variance in the reported ppm level. See, for example, FIG. 13. Exacerbating this effect is the fact that the presence of even small amounts of cyanuric acid can reduce the signal output by several tens of millivolts, Another difficulty encountered in using an ORP sensor to measure sanitizer level is that it is highly pH-dependent—on the order of 50 mill volts per pH change. Therefore, the output of ORP sensors in an automated system frequently requires adjustment, which is clearly undesirable from an operational standpoint.
Another type of chlorine sensor is a galvanic sensor, which is based on the galvanic coupling of dissimilar metals through electrode depolarization (See U.S. Pat. No. 3,413,199 to Morrow and U.S. Pat. No. 2,382,734 to Marks). Depending on the metals involved, a measurable current that is linearly proportional to the amount of free chlorine in ppm flows between the electrodes. See FIG. 14. The current flow is due to the anode being oxidized and chlorine being reduced at the cathode. These reactions proceed as long as there is a source of chlorine being reduced.
This technique requires a constant flowrate of water past the electrodes to replenish the chlorine being reduced at the cathode. Since the current flow is linear, the signal can be more easily calibrated to specific amounts of chlorine as opposed to the logarithmic curve from the ORP sensor. While the signal is also somewhat sensitive to pH, in a limited pH range between 6.5 and 8.0, the effect can be compensated for electronically or by software. Nevertheless, if the pH varies over a wide range or is unstable, a buffering agent may need to be added to the sample to stabilize the pH in the range of measurement.
The signal measured in a galvanic sensor also is sensitive to temperature, requiring further compensation, but the compensation usually is easily achieved in the normal range of temperatures encountered in pools and spas (generally about 65.degree. F. to about 110.degree. F.).
The type of electrode employed in such sensors is much less affected by the presence of cyanuric acid than are those of the DPD sensors; however, as noted previously, it will also indicate less chlorine with respect to the DPD measurement in systems with cyanuric acid. Nevertheless, it will not display the wide deviations found in ORP measurements. The impact on automated systems is minimized by the fact that the dosage set point can be reasonably set at a level of 1-3 ppm without causing wide fluctuations in dosage.
In any event, galvanic sensors also suffer from several drawbacks that also make their use problematic. For one thing, since the reaction is galvanic, the anode must be an electrically active metal and so it becomes oxidized over a period of time, which can lead to chemical changes on its surface that effect its sensitivity to chlorine. Also, depending on the level of electro-activity, the anode may begin to plate onto the cathode, further reducing the sensitivity. Therefore, some abrasive element such as corundum is commonly added into the flow cell to mitigate the accumulation of reactant byproducts on the electrode surfaces. Without this abrasion mechanism, the output from the sensor diminishes over a fairly short time—typically 1 to 2 weeks.
Galvanic sensors are also affected by the electrolytic conductivity of the water, usually requiring it to be below about 1000 micro-Siemens or about 500 ppm. This precludes their use in pools that use a salt chlorinator to generate chlorine electrolytically from sodium chloride added to the water. The conductivity in this water is on the order of about 6000 mu·S or about 3000 ppm. Although this type of sensor is superior to the ORP measurement, it still does not address the practical aspects of using it for long-term, unattended operation in pools and spas sufficiently.
Other types of sensors used in the detection of chlorine are amperometric in nature. Amperometric sensors use an applied bias voltage to establish a current flow based on the level of free chlorine. The detection mechanism is based on electrode depolarization, which is the same as that involved in galvanic sensors except that, since there is an applied voltage, the electrodes can be of the same material. The primary issues raised by amperometric sensors are corrosion resistance and catalytic activity. To address these issues, noble metals such as platinum or gold tend to be used for the electrodes. Noble metals do not display the oxidation effects to the degree seen with the galvanic sensor and are not particularly sensitive to electrolytic conductivity Although any electrochemical sensor will require some minimal amount of conductivity to function, this is not a problem in pools since the conductivity of most tap water is at least around 300 ppm.
The difficulties of amperometric measurement in pools and spas are primarily related to passivation of the anode and deposits of foreign matter on it. Various methods have been employed to in attempts to avoid these effects to certain degrees. Thus, Wallace, in U.S. Pat. No. 2,350,378 and Morrow, in U.S. Pat. No. 3,413,199, disclose including some abrasive mechanism in the flow cell to keep the anode clean and electrically active. The difficulty with use of an abrasive, however, is that the abrasive nature of the cleaning wears away electrode material. The abrasive material also needs periodic addition or replacement and can require a tedious process of disassembling the flow cell.
Other techniques for protecting the integrity of the electrodes use a membrane-covered anode or a membrane covering both the anode and the cathode and enclosing them in an electrolytic solution. The membrane is permeable to the chemical that is the subject of the measurement, allowing the chemical to pass through the membrane and, due to the combination of the bias voltage and electrolyte, is reduced at the cathode.
These arrangements, however, also suffer from several drawbacks. For instance, they require periodic refilling of the electrolyte solution within the membrane and sometimes complete disassembly to replace the membrane or for other cleaning and maintenance. Moreover, as with the galvanic sensor, measurement also sometimes requires pH-buffering to stabilize and or to reduce the pH of the water to be tested to a certain level.
Other types of chlorine electrodes have been fabricated without membranes, but they require a buffer solution to adjust the pH to a set value for the measurement to be valid. One such arrangement, discussed by Nakagawa in U.S. Pat. No. 3,902,982, uses bare electrodes housed in a chamber that is fixed to a motor that stirs, at a fixed rate, the solution to which is added a buffer solution. This system not only uses a buffer but requires a sample to be taken at various times precluding its use in any inline monitoring application. Morrow, in U.S. Pat. No. 3,959,087, teaches an electrode assembly wherein both electrodes comprise copper and wherein a buffer solution is not required between pH 5 and pH 9. In practice, however, this arrangement has been found to be very susceptible to the amount of chloride-based salt in the water. Many pools in use today use chlorine that is generated on site by sodium chloride salt added to the water which is converted to chlorine through an electrolytic cell. The conductivity in these systems is quite high, such as about 6000 .mu·S or about 3000 ppm. In addition, this arrangement has been found to exhibit random steps in the electrical response to the addition of chlorine or when fresh water is added to the system.
Still other methods involve using electrical pulses of various levels and polarity to de-plate contaminants from the electrode surface. These methods are based on the specific contaminants that are to be removed. Outside of a laboratory setting with known contaminants, the effects of electrical cleaning can be random and even detrimental to the measuring process. Considering the wide range of possible contaminants in pool water, therefore, this method is not practical.
Corrosion resistance and catalytic activity considerations for the anode of the sensing cell are somewhat differently than they are for the cathode. The anode functions primarily to supply electrons in the reduction of chlorine at the cathode. As such, the primary requirements with regard to the anode are electroactivity and corrosion resistance. The cathode, in contrast, is where the chlorine is reduced, and noble metals are more catalytically active in this regard. However, although noble metals are quite corrosion resistant, corrosion resistance is not the ultimate criterion for selecting a cathodic material. For example, titanium is among the most non corrosive metals known but it passivates quickly, almost completely eliminating any current flow.
For those sensors in which a bias voltage is applied, the bias voltage chosen is based on the oxidizing or reducing potential of the chemical of measurement. The sensor signal exhibits a plateau of limited current at the characteristic reduction potential of the element. In addition, the diffusion is rate limited because, at the reduction potential, the entire amount of oxidizer present is reduced at the cathode. However, in water environments common to pools and spas, the conditions for diffusion limited current are not very favorable, as noted by Marks and Bannister in Amperometric Methods in the Control of Water Chlorination, Analytical Chemistry, Vol. 19, No. 3, pp. 200-204 (1947). However, as noticed by Marks and Bannister, even though there is not necessarily a diffusion limited current, the current still maintains a linear relationship to the level of chlorine based on the applied bias voltage and the concentration of oxidizing compound in the normal range of oxidizer residuals.
With respect to pH sensors, many of those known in the prior art are based on the H.sup.+ sensitivity of certain microporous glass mixtures. The glass is highly selective toward H.sup.+ ions and responds quickly to changes in pH. The sensing portion of the sensor is usually mounted in a common housing that also contains a reference electrode. The reference electrode provides a stable electrochemical interface to the test solution and completes the electrical circuit providing the signal.
The reference electrode typically contains an equilibrium mixture of sodium or potassium chloride, silver chloride and a silver wire that has a coating of silver chloride. The reference electrode is coupled electrolytically to the sensing portion by a porous junction, typically of microporous Teflon™. The electrolytic coupling is maintained by a small leakage of the internal fill solution through the porous junction to the solution being measured.
Such electrodes suffer from several drawbacks. The glass-sensing portion is of extremely high impedance, typically on the order of 100-500 megohms, thus requiring a very high input impedance amplifier to measure the signal properly. The reference electrode also has impedance associated with it, based on the leak rate of the internal fill solution. Therefore, the reference electrode must provide a sufficient leak rate to minimize the impact on the total electrode impedance. The leak rate of the reference electrode, however, directly effects the lifetime of the sensor. The high input impedance also makes the system very susceptible to noise and grounding problems. In addition, the electrodes are quite delicate due to the fragile nature of H.sup.+ sensitive glass, which also is very susceptible to fouling by organic contaminants, especially those found in pools. Such sensors also are quite expensive and have a relatively short life, especially in inline systems exposed to constant flow. More rugged versions of the probes are available that attempt to protect the glass sensing surface and contain larger amounts of internal fill solution, but this adds even further to the cost.
Other pH sensors of various sensing mechanisms are also known. For example, some sensors are based on ionically sensitive field effect transistors (ISFET). The gate of the transistor is coated with a pH sensitive polymer. The transistor is biased in such a way that when the gate is exposed to changing pH, a voltage change on the circuit output is caused. The main difficulty with these sensors is they require a specific meter that is calibrated for each sensor. Further, these sensors are extremely expensive, which has limited their widespread acceptance.
Other pH sensors are based on various pH-sensitive polymers coated onto an electrical substrate. Such sensors are used mainly in a laboratory environment and have an extremely short lifespan in a pool environment due to incompatibilities of the polymer coatings with the chemicals used to treat the water.
Antimony pH electrodes also are known in the prior art. Antimony pH sensors employ a piece of monocrystalline antimony because it has been known to have a more reproducible pH response. See, for example, U.S. Pat. No. 4,119,498 to Edwall et al. However, such sensors have been found to suffer particular drawbacks when applied to swimming pool measurements. For instance, since the antimony electrodes are composed of crystaligraphically-oriented antimony, they are very small and flat with almost a mirror-like surface. This small size results in a relatively high electrical impedance and very rapid oxidation in even mild oxidizing environments. The mirror surface also does not lend itself to any sort of protective coating due to problems with adhesion.
Other antimony pH sensors use polycrystalline rod material with various methods used to seal the interface between the antimony and the sensor housing (see U.S. Pat. No. 4,681,116 to Settler, and U.S. Pat. No. 3,742,594 to Kleinberg). However, these electrodes do not address the oxidation issues and also suffer from all the attendant difficulties associated with bare antimony.
The subject inventor's U.S. Pat. No. 6,653,842 described a pH sensor based on an antimony/antimony oxide electrode. The reference electrode of that patent was a bare metal electrode as described therein. The pH sensor of that patent was based on the galvanic potential between the antimony and metal reference electrode. In water at relatively low levels of chlorine (less than 3 ppm), the electrode is minimally affected by the presence of the chlorine, but at levels above that, the oxidizing effects of the chlorine start to modify the electrode surface of both the antimony and the metal reference electrode. Surface scans of the electrodes show that that the largest part is composed of antimony of several electron states as well as the effect of high chlorine (5 ppm) on the antimony. From the photomicrographs and scans, it can be seen that, after exposure to a high chlorine environment, the surface of the antimony has less pure antimony than it had originally and the relative amounts of carbon and oxygen have increased. Also as can be seen from EDA scans of the zinc reference electrode, the antimony has been deposited on its surface. The effect of this plating is to reduce the galvanic potential between the antimony and zinc, causing a shift in signal offset and a reduction in sensitivity to pH changes. The sensors, therefore, require abrasive cleaning before the response returns to normal. Since abrasive cleaning techniques are not suitable for consumer use, this electrode would not be suitable for long-term measurement systems where the chlorine concentration is high (above about 2 ppm).
Various solutions have been proposed to address the problem of electrode erosion and degradation. For example, U.S. Pat. No. 4,818,365 to Kinlen proposes using dip coatings of Nafion. The methods of Kinlen, wherein he dip coated electrodes in a Nafion solution and cured and then annealed them, was investigated. It was found with respect to antimony that the coatings resulting from this process were not only highly variable regarding their pH response but also retained insufficient adhesion and uniformity to the metal surface.
A major difficulty in using dipped and cured coatings of Nafion on antimony is that the curing process has a difficult process endpoint based on time in the oven and its temperature as noted by Kinlen. In other words, insufficient heat or time or too much heat or time in the oven has been found to have dramatic effects on the resultant coatings.
An additional difficulty in pH sensing is that the degree of adhesion of the coating to the electrode greatly affects the relative sensitivity of the electrode with respect to pH changes. This aspect of adhesion regarding sensitivity was not as pronounced with the chlorine sensor in that the primary requirement of the anode was to supply electrons wherein the electrical resistivity was the principal issue. In the case of the pH electrode the principal issue is the sensitivity to hydrogen. Nafion acts as a barrier to chlorine while allowing hydrogen to pass through. However there exists an impedance gradient between the inside surface of the membrane and the metal surface with regard to hydrogen sensitivity. Depending on the degree of adhesion this gradient is highly variable resulting in a range of sensitivities that are fairly random. Also affecting the sensitivity is the thickness of the membrane; the thicker the membrane the higher the resistance to the passage of hydrogen ions. Photomicrographs of a Nafion dip coating have shown the difference in coating thickness on different areas of the same antimony electrode. The electrode surface showed a blistered appearance after a time in water. This difficulty was also referred to regarding the chlorine anode earlier.
Mosier et al patent application Ser. No. 10/848,196 teaches a method of using Nafion tubing around a platinum anode coupled with a porous titanium cathode with a Nafion membrane over the end exposed to water. The electrodes then are subjected to an extremely high voltage (.about.1500V) and used to drive an Electro Kinetic pump. The driving force electrolizes the water between the electrodes that generate hydraulic pressure and in the process produce air bubbles that limit long term stabile operation of the pumps. Therefore, to eliminate the gas bubbles generated during electrolysis, Mosier et al. provide and air vent at the end of the anode exposed to air. The exposure of the anode to oxygen is detrimental to the long term functioning of the electrode used in sensing an amperometric parameter as in chlorine/bromine measurement.
Sensors for measuring the total dissolved solids (TDS) in water are used to measure the amount of dissolved salt in pool and spa environments, as well as in many other aqueous environments. TDS is related to the conductivity of the water and is a useful measurement in that there are guidelines related to the maximum amount of dissolved solids that are allowed in drinking water and in pool/spa environments. In addition, certain conditions in a swimming environment are sometimes difficult to diagnose without knowing its TDS level. TDS also can interfere with the effectiveness of the sanitizer, allowing algae formation even when the sanitizer is at the recommended level. Moreover, chlorine is generated on site for pools that use dissolved salts in the water. In such cases measurement of TDS is needed because the chlorine generators require a minimum amount of dissolved in the form of salt in the water for the chlorine generator to function properly. However, generally, the instrumentation to measure TDS is based on special test strips or costly meters and probes.
Thus, there is a clear need in the marketplace for an efficient low cost reliable long-term measurement system for measuring pH, halide (e.g., Cl or Br) concentration, temperature and total dissolved solids (TDS) to allow wider usage in smaller residential pools and spas as well as in commercial pools or other high end residential applications and, in particular, a such a system that eliminates the need for frequent cleaning.